This relates generally to the field of semiconductor devices and processes, and more specifically to the structure and fabrication method of microelectromechanical system (MEMS) devices in leadframe-based pre-molded packages with acoustic channel suitable for microphones, speakers, and pressure gauges.
The wide variety of products collectively called microelectromechanical system (MEMS) devices are small, low weight devices on the micrometer scale, which may have mechanically moving parts, including transducers and actuators, and may have parts sensitive to thermal, acoustic, or optical energy. Because of the moving and sensitive parts, MEMS devices have a need for physical and atmospheric protection. Consequently, MEMS devices are typically placed on a substrate and surrounded by a housing or package, which shields the MEMS device against ambient and electrical disturbances, and against stress.
A MEMS device integrates mechanical elements, sensors, actuators, and electronics on a common substrate. MEMS device manufacturing aims at using batch fabrication techniques similar to those used for non-MEMS microelectronics devices. MEMS devices can thus benefit from mass production and minimized material consumption to lower the manufacturing cost, while trying to exploit the well-controlled integrated circuit technology, although today the production of MEMS devices is still far from the level of maturity that manufacturing and packaging of microelectronics have reached.
MEMS devices may be mechanical sensors, both pressure sensors such as microphone membranes, and inertial sensors such as accelerometers, either of which may be coupled with the integrated electronic circuit of the chip. Mechanical sensors react to and measure pressure, force, torque, flow displacement, velocity, acceleration, level, position, tilt, and acoustic wavelength and amplitude. Among the general requirements for pressure sensors are long term stability, small temperature sensitivity, low pressure and temperature hysteresis, resistance to corrosive ambients, and often hermeticity. Two specific bulk micromachining processes employed in MEMS sensor production to create in bulk semiconductor crystals the movable elements and the cavities for their movements are anisotropic wet etching and deep reactive ion etching (DRIE).
The semiconductor MEMS pressure sensors and microphones are assembled on insulating substrates, which include multi-level metallizations patterned for horizontal and vertical connections. As an example, the substrate may have four metallization levels. The assembly typically involves adhesive chip attachment and metal wire bonding to the terminals. In many products, the terminals are positioned similar to the compact Small Outline No-Lead (SON) or Quad Flat No-Lead (QFN) type semiconductor devices, since they show the compact outline of chip-size packages. The substrates also include the air channels needed to expose the membranes to the ambient pressure and acoustic signals to be monitored by the MEMS devices. For protection and robustness reasons, the chip, wires, and portions of the substrate are packaged in a metal can or molded in a plastic encapsulation. As an example, the dimensions of a MEMS cuboid analog-output microphone with four terminals for mobile phones may be about 4.8 mm by 3.8 mm by 1.25 mm, or even as small as 2 mm by 2 mm by 1.25 mm (resulting in a 4 mm2 footprint).